It is known that in the production of plastic foils which can be wound to form a coil, thickness-error profiles can be detected for the foil across the width, of which systematic plus thickness errors and systematic minus thickness errors can be discerned. The term "systematic" is here used to refer to errors in the thickness of the foil that arise as a function of some property of the foil-fabricating system.
Such errors can develop in association with certain parts of the manufacturing system and, consequently, when a foil having such systematic thickness errors is wound in a coil, the plus thickness errors, i.e. thicknesses greater than a desired or set point value of the foil thickness, tend to accumulate in the coil to form annular bulges therein. Conversely, minus thickness errors or thickness deviations from the standard or set point value, which are such that the thickness in a particular region of the foil is less than the set point value, can accumulate to form annular troughs in the coil.
The system-determined thickness errors may result, for example, from tool or extrusion die tolerances.
Systematic thickness errors are largely unavoidable and appear in the cross section profile of the foil produced and in a stationary state of the apparatus for producing the plastic foil, always arise at the same location across the width of the foil. If efforts are not made to avoid the build up of bulges and troughs in the coil, the result is a barrel-shaped coil with annular bulges and troughs as described above.
It has been proposed, because of this problem, to avoid the addition of thickness errors as the foil is wound into a coil. This can be achieved by shifting the thickness-error profile over the width of the plastic foil or film so that there are no locations on the coil where there are special build ups of the errors. The coil thus appears to be free from visible addition of thickness errors. In this case, the thickness errors are shifted by and large from layer to layer to different locations along the length of the coil.
To achieve this in the production of so-called blown film, in which a thermoplastified tube is generated by extruding the thermo-plastified synthetic resin through an annular nozzle, the tube is then blown to form a balloon and the balloon is flattened to form a web which is wound into the coil, either the blowing head with the extrusion tool or die must be given an angular periodic oscillating movement or the so-called flattening device must be correspondingly periodically moved. In some cases, both the blowing head and die and the flattening unit are moved in an angularly oscillating sense.
An apparatus in which the flattening device and the blowing head or both are oscillated is of expensive construction, and is not always capable of satisfactory results since the coil has irregularities, although these are not always in the form of the annular bulges and troughs described. In wide-mouth dies from which a flat web of the foil or film is extruded directly, i.e. so-called slot-nozzle dies, the magnitude of the thickness errors can be reduced and the thickness-error profile leveled in response, for example, to a measured-thickness profile, by adjusting the gap width of the wide-mouth die. The superposition and distribution of singular thickness errors is also known in this context.
This technique does reduce the thickness tolerance although in practice it is found that there is a residual detrimental thickness profile with error addition in the coil.
The above-mentioned patent application describes a process for achieving a cylindrical contour at the end of coiling of the foil with significantly greater precision when an error distribution is no longer carried out.
This system allows the elimination of moving structures in units hitherto required for error distribution and thus represents a significant simplification. This method is characterized by a measurement of the thickness-error profile of the produced plastic foil with its systematic plus thickness errors and/or minus thickness errors before the foil is wound into the coil.
According to this method, a number of n layers of the foil with the systematic plus thickness errors and/or minus thickness errors are wound in a coil such that n=1 or n is .gtoreq. 1 and thereafter a number of layers with inverse thickness-error profile are wound on the coil. The process is repeated alternately between layers of the one thickness-error profile and layers of the inverse thickness-error profile until the end of the coil with a sufficiently exact cylindrical contour is reached.
This process results in a coil with a very precise cylindrical contour in which the individual layers are packed firmly together and which is practically free from interlayer spaces and which, moreover, is free from air inclusions. It is especially advantageous with thin plastic films.